Comment.module does not clean up it's va

It is a feature that modules don't delete their variables on disabling of that module, that way data can be kept while the functionality is (temporarily) disabled.

However, modules should delete their variables when uninstalling though, and this is not possible for the Comment module. In fact, the Comment module doesn't have a comment_uninstall() function so that is definitely something we'd have to look at. It is best to change this in D7 first, and then port back to D6.

Better handling of variables & their defaults is addressed in #145164: DX: Use hook_variable_info to declare variables and defaults (as is automatic deletion of variables).

Attached is a first step. This does not remove the {comment} and {node_comment_statistics} tables, because I'm not 100% that those can be deleted safely.

The drupal_set_message() in here looks like left over debug, otherwise looks good to me.

Probably be much easier to do:

  $settings = array(
    'comment',
    'comment_default_mode',
    'comment_default_per_page',
    'comment_anonymous',
    'comment_subject_field',
    'comment_preview',
    'comment_form_location',
  );

  foreach (array_keys(node_get_types('names')) as $node_type) {
    foreach ($settings as $setting) {
      variable_del($setting . '_' . $node_type);
    }
  }

  variable_del('comment_block_count');

Fixed in #361130: Uninstall comment module variables. Sorry for the duplication.